1. Field of the Invention
The present invention relates generally to a process for converting biorenewable feedstocks into fuel, and more particularly, not by way of limitation, to a process for removing phosphorus from biorenewable feedstocks, such as vegetable oils, animal fats, and other sources of fatty acids/glycerides, while converting biorenewable feedstocks into fuel and chemical products and intermediates. The inventive method may be adapted to recover the removed phosphorus.
2. Brief Description of the Related Art
Biomass is a renewable alternative to fossil raw materials in production of liquid fuels and chemicals. Increase of biofuels production is part of the government's strategy to improve energy security and reduce green house gas emissions. However, most biomass has high oxygen content which lowers fuel quality and heat value. Upgrading biomass or biomass intermediates into high quality hydrocarbon fuels thus requires removal of oxygen. The biomass oxygen may be in the form of an ester, carboxylic acid or hydroxyl groups.
Removal of oxygen by catalytic reaction with hydrogen is referred to as hydrodeoxygenation (HDO). This reaction may be conducted with conventional fixed-bed bimetallic hydrotreating catalysts such as sulfided nickel-molybdenum (NiMo) or cobalt-molybdenum (CoMo) which are commonly used in refineries.
Unrefined vegetable oils and animal fats have several hundred ppm phosphorus in the form of phospholipids. Phosphorus is a key nutrient for plants and the animal life they support. Fertilizers contain phosphorus (in phosphate form) to maintain soil nutrition and support growth. In an isolated ecosystem, such as a forest, the phosphorus taken from the soil to nourish the growing vegetation releases back into the soil after decomposition of dead plants. However, in societies dependent on modern agriculture, the phosphorus cycle is not closed.
Phosphorus, mined from rocks, is converted to fertilizers. The fertilizer is added to the soil and converted into the growing plant's cellular makeup. The plant is eaten by animals and humans alike and, what is not incorporated into the body to support growth (bone, cell membranes, etc.), is washed away in urine, feces, and fertilizer runoff. Phosphorus, thus, leaves the soil and ultimately ends up in water ways where it has a negative impact on aquatic ecosystems.
Phosphorus discharged to water systems contributes to algal blooms. Algae from this type of phosphate discharge grow rapidly, cover wide sections of the water system, and then die. As the algae decompose, they consume the water system's oxygen, starving fish and native plant life. The flow of phosphorus from mine to water systems, where it remains essentially unrecoverable, has raised questions about sustainability of our modern agricultural practices. In an article entitled “Peak Phosphorus” which appeared in the April 2010 issue of Foreign Policy, James Elser and Stuart White warn of depleting phosphorus mines, and discuss the significance of America's transition from a major phosphorus exporter to a country that needs to import more than 10% of its phosphorus today.
The emerging biofuel industry is expected to increase phosphorus demand and potentially aggravate the situation. For example, algae, the touted green crude of the future, need phosphorus to live up to their potential as high oil yielding energy crops.
Biofuels themselves need to be devoid of phosphorus since the latter tends to form harmful deposits on engine parts. The ASTM D6751 standard for biodiesel sets a maximum specification limit of 0.001 wt % for phosphorus. A similar low phosphorus requirement exists for most bio-based chemicals (e.g. oleochemicals).
Phosphorus in vegetable oils and animal fats is mainly in the form of phospholipids. This class of compounds is characterized by a diglyceride, a phosphate group, and a simple organic molecule such as choline. Phospholipids are a key component of most vegetable oils and animal fats. Crude soybean oil, for example, can contain up to 2.5% phospholipid.
Removal of phosphorus from vegetable oils is referred to as degumming. Degumming typically involves contacting the oil with phosphoric and/or citric acid followed by separation of the resulting solids/sludge. Typically, a high-shear mixing device is used to achieve efficient contact between the aqueous acid phase and the oil phase. Solids comprising phospholipid and phosphate/citrate salts of calcium/magnesium (also present in crude vegetable oils and animal fats) are thus formed and allowed to grow via flocculation in a holding/residence tank. Centrifugation is commonly used in the subsequent step to separate the phosphorus-rich solids/sludge from the degummed oil.
An alternative to the aforementioned acid degumming process is physical degumming or dry degumming. This involves adsorption of polar compounds (including phospholipids and soaps) on active clay. Both natural (e.g. Fuller's earth) and synthetic (e.g. Grace Davison's TriSyl®) clays may be used. The physical degumming process typically involves injection of the powder clay into an agitated slurry tank. The spent clay—containing adsorbed phosphorus and metal compounds—is then separated using a pressure leaf filter. The dry degumming process is also referred to as “bleaching” since the active clay removes color bodies in addition to phosphorus and metals.
Most modern vegetable oil refining processes use both chemical and physical degumming to produce Refined, Bleached, and Degummed (RBD) products. RBD grade vegetable oils are characterized by their low phosphorus and metals content, good shelf life, and light color. Such vegetable oils are the preferred feedstock for biofuels produced by transesterification and hydroprocessing.
Hydroprocessing—specifically hydrodeoxygenation followed by hydroisomerization—is considered to be a more attractive method for production of bio-based diesel and jet fuel than transesterification. (Diesel from hydroprocessing of fats and oils is referred to as “renewable diesel” and “green diesel” to distinguish it from fatty acid alkyl ester “biodiesel.”) Transesterification produces alkyl esters, which are absent in crude oil and therefore do not meet fuel specifications developed for petroleum fuels. As such, alkyl esters are only marketed as fuel additives or blendstock. On the other hand, hydroprocessing produces bio-based hydrocarbons that represent the most desirable molecules found in petroleum diesel and jet fuel—namely n-paraffins and iso-paraffins. These hydrocarbons are considered desirable for diesel because of their clean burning properties (low particulate matter emissions and engine deposits) and high cetane values. The conversion of fatty acids to n-paraffins is given by Eqs 1 and 2 for the illustrative case of oleic acid conversion to n-octadecane and n-heptadecane.HOOC—C17H33+2H2→n-C18H38+2H2O  (1)HOOC—C17H33+H2→n-C17H36+CO2  (2)
In the case of mono-, di-, and tri-glycerides, the glycerol backbone is converted to propane. Hydroisomerization converts the straight-chain n-paraffin to branched iso-paraffins that have better low temperature properties (e.g. lower cloud point). Renewable diesel is thus considered a “second generation” or a “drop-in” biofuel.
Hydroprocessing catalysts typically include one or more base metals or noble metals supported on an alumina- and/or silica-containing support. These catalysts are designed to achieve one or more of the following hydrogenation and hydrogenolysis reactions: deoxygenation, desulfurization, denitrogenation, demetalation, hydrocracking, hydroisomerization, and saturation of double bonds. The catalyst is typically in the form of extrudates, shaped-extrudates (three- or four-lobed extrudates), Raschig rings, or shaped tablets, for fixed-bed reactors, or in powder form for slurry reactor systems. Once the hydroprocessing catalyst is deactivated or fouled such that it cannot achieve the desired conversion/throughput, it is discharged from the reactor. In most cases, this spent catalyst is then sent for catalyst regeneration/reactivation or metals recovery. The regeneration/reactivation and metals recovery process generally includes a pyrolysis step, where hydrocarbons are volatilized in the absence of oxygen, and a decoking step where the carbon deposited on the catalyst is burned off under a controlled oxygen atmosphere.
The phosphorus removed during conventional degumming operations is incorporated into waste streams—sludge from acid degumming and spent clay from physical degumming. Since there is nothing to “reclaim,” the sludge and spent clay are commonly land-filled. As such, the prior art offers no opportunity for recovery of phosphorus removed from vegetable oils and fats.
To this end, there is a need for a process that can trap phosphorus during conversion of fats and oils to biofuels and bio-based chemicals, such that the trapped phosphorus can economically be recovered for reuse (e.g. as fertilizer). It is to such a process that the present invention is directed.